Conventional X-ray tubes for high-power operation typically comprise an evacuated chamber (tube envelope) which holds a cathode filament through which a heating or filament current is passed. A high voltage potential, usually in the order between 40 kV and 160 kV, is applied between an electron emitting cathode and the tube anode. This voltage potential causes the electrons emitted by the cathode to be accelerated in the direction of the anode. The emitted electron beam then impinges on a small area (focal spot) on the anode surface with sufficient kinetic energy to generate X-ray beams consisting of high-energetic photons, which can then e.g. be used for medical imaging or material analysis.
X-ray tubes of the rotary-anode type were first built in the 1930s. Compared to stationary anodes, a rotating anode offers the advantage of being able to distribute the thermal energy that is deposited onto the anode target's focal spot across the larger surface of a focal ring (also referred to as “focal track”). This permits an increase in power for short operation times. However, as the anode disk is now rotating in a vacuum, the transfer of thermal energy to the outside of the tube envelope is not as effective as the liquid cooling used in stationary anodes. Rotating anodes are thus designed for high heat storage capacity beneath the focal track and for good radiation exchange between the anode disk and the tube envelope. A minimum diameter of the anode disk of between 80 and 240 mm is needed, which gives rise to a slight wobble of up to approximately 0.05 mm. This is significant in relation to an optical focal spot size of down to 0.15 mm (in a projected view as seen from the X-ray detector of an X-ray system which comprises said X-ray tube).